A tag serves to identify the thing to which it is attached. Many products today are identified with RFID tags. The RFID tag has gained popularity as a way to identify products because it speeds checkout processing and aids automatic inventory management. The RFID tag is attached to an object and then scanned or “interrogated” using radio frequency electromagnetic waves emitted from an interrogator. Interrogating the RFID tag with radio wave allows the interrogator to be out of direct line-of-sight of the tagged item and to be located at a greater distance from the item than is permitted with other approaches such as optical scanning.
Typically, an RFID tag system includes one or more interrogators and an RFID tag for each item to be tracked. The interrogator includes a radio transmitter to send signals to the RFID tag and a radio receiver to receive signals sent back from the RFID tag. The interrogator can also typically be connected to a network so that the information from the various RFID tags can be centrally gathered and processed.
The RFID tag typically includes an antenna and an integrated circuit chip. Depending upon the radio frequency, the antenna can be anywhere from just a few turns to a few hundred turns of wire. The integrated circuit chip can include the radio transmitter and receiver functions along with data storage. The data stored on the chip can range from a simple product identifying number to extra identifying data to further identify the object itself. It is also possible for data to be written into the chip from various interrogators. For example, the location history of a product can be written into an RFID tag as the tagged product is moved from the store room to the sales area and perhaps to other associated retail outlets.
RFID tags are typically classified as either active or passive. Passive tags derive their energy from the interrogating radio signal and are generally limited in application to product checkout where the tagged item can be placed in proximity to the interrogator's antenna. Active tags contain a small battery as an energy source and can broadcast a radio signal over a greater range.
Tracking tagged items with RFID is valuable to retailers because it reduces manual receiving and inventory management procedures. Products can be tracked automatically from distribution centers to storerooms and from storerooms to the store's retail area. Interrogators in the retail area can provide real time indication of low stock or misplaced items and speed customer checkout. For example, the store clerk can accelerate the checkout process as they are not required to individually process each of the items a customer brings to the checkout counter. Simply placing the items in the vicinity of the interrogator is all that is typically needed to interrogate the RFID tags and checkout items.
It is also desirable to know the precise location of a tagged object. Range to a tagged object can be estimated by measuring the propagation time of a radio signal sent to and from a tag. Further accuracy can be obtained by subtracting the known circuit delays of the interrogator's transmitter and receiver along with those of the RFID tag.
Even with multiple conventional interrogators operating, finding a precise tag location to within a few centimeters is problematic. Expensive hardware is needed to accurately process radio waves traveling at the speed of light. For example, radio waves, which propagate at approximately 30 centimeters per nanosecond, require a 1-gigahertz counter to locate an RFID tag to within 30 centimeters. Less expensive counters operating at lower frequencies have a higher degree of granularity and suffer from even greater uncertainty in precisely locating a tag.
Unfortunately, circuitry operating at gigahertz rates is not only expensive, but typically consumes a lot of power and is difficult to design. Power consumption remains a problem in conventional RFID tag systems. Constant monitoring of a tag may identify the location of a tag but it will eventually dissipate the tag's battery. A dead battery in a tagged item is tantamount to a missing item. Replacing tag batteries is costly in terms of both labor and materials and is generally not an option.
Accordingly, there is a need for a system to precisely determine the positions of a number of RFID tags while addressing the aforementioned problems.
Like reference numbers and designations in the various drawings indicate like elements.